Each mass containing body generates a gravitational field which effects an attractive force with respect to other masses. In case of a spherical body having a homogeneous mass distribution, the gravitational field emanating from its surface is proportional to the sphere's diameter and its density. Outside the body, the field strength of its gravitational field decreases with the reciprocal value of the square of the distance to the sphere's centre. The gravitational field of the earth, acting as fall acceleration, at the earth's surface has a magnitude of about 9,81 meter per square of a second. A spherical volume of water of 1 m diameter at its surface creates an intrinsic gravitational field obeying a relation of 1 to 70 000 000 as compared to the earth's gravitational field.
This value is extremely small, but may nonetheless be measured by means of a gravimeter instrument. These devices usually consist of a precision mechanical spring mass system, upon which the gravitational field to be measured acts. In this respect, the spring is lenghtened by a weight force of the mass element proportional to gravity, and the change of length thereof, or a necessary compensation force for reaching a defined reference length is taken as a measure for the acting gravitational field strength.
Gravitational pendula, floating bodies and falling body assemblies are also applied for gravitational field measurement.
Besides scientific applications like the measurement of the tidal forces generated by moon and sun, gravimeter instruments are primarily applied for the search and exploration of mineral resources like petrol, gas, coal, ores and salts. In this respect, there is made advantage of the fact that these materials usually have a density different from the normal surrounding stone, and/or modify the density thereof, respectively, when contained in the stones pores.
In the region to be examined, a gravimeter instrument is moved along the earth's surface, and the gravitational measurement values as well as the geographic position thereof are recorded. The gravimeter may also be mounted to an aircraft flying above the region to be examined. Further, it is common to lower gravimeter instruments into bore holes, and to record the measured gravitational field values and the corresponding depth values during the lowering operation.
The result of these series of measurement are maps of the geologic gravitational field abnormalities, and/or depth diagrams showing the field in the subsoil, respectively.
On behalf of the created maps and diagrams, the experienced geologist can now detect density differences present in the subsoil, and can thereby draw conclusions concerning the presence of deposits and the exploitation worthiness thereof.
U.S. Pat. No. 6,612,171 B1 discloses a gravitation measurement apparatus for measuring the gravitational field in bore holes, in order to detect the density of formations in the subsoil. Therein, in order to perform a difference measurem ent, the disclosed gravitation measurement apparatus may be moved between two positions.
DE 689 15 45 T2 discloses a gravitation gradiometer. Therewith, out of diagonal components of a gravitational gradient tensor are deemed to be measured. In particular, a special bending rotational bearing is disclosed.
WO 98/57197 also discloses a gravitation gradiometer. Therein is described, that the gravitation measurement is often performed out of airplanes, to detect petrol deposits. Accelerations occurring in the aircraft and thereby affecting the gravitation gradiometer are deemed to be compensatable in the gravitation gradiometer described in said pamphlet.
All the known gravitation measurement devices are not adapted for the use as industrially capable sensors for detecting material amount and/or for detecting a filling level of a filling good in a container, respectively. To the one hand, they are much too expensive, too voluminous, and only give the gravitational field in one spatial dimension. The outputted gravitational field display has to be interpreted by a skilled person in order to be able to derive further information therefrom.
Further, response time and energy consumption of the known gravitation measurement devices are much too high for an industrial filling level sensor.